1. Field of the Invention
The invention relates to a magnetic disk substrate with a core of synthetic material.
2. Description of the Prior Art
U.S. Pat. No. 3,761,333--Kleinbeck et al shows a magnetic recording disk with a core made of synthetic material. According to this arrangement, a magnetic disk is made by placing a magnetizable foil into one half of a mold, and then placing a prefabricated core of synthetic material on the foil. Into the other half of the mold a second magnetizable foil is placed. Under the influence of heat and pressure, the two magnetizable foils are molded with the core of synthetic material. This patent also teaches the production by the above-described method of pure magnetic disk substrates consisting of synthetic material. The magnetic layer is applied in a subsequent process step onto such substrates.
One problem in the production of magnetic disks and magnetic disk substrates, in accordance with this prior art method is that when the finished magnetic disk or disk substrate is removed from the mold, parts of the disk or substrate adhere to the mold. This causes surface deformations or defects in the magnetic layer. A particularly serious problem is that from a specific temperature upward, the so-called blocking point, synthetic foils and materials are blocking or adhering, even if the pressure applied is very low. This can be countered with anti-blocking agents, but their effect is to increase the surface roughness in order to reduce the mold-contacting surface. Although the introduction of separating agents between parts of the mold and the foils or the synthetic material core has an improving effect, tests have shown that it is far too small to satisfy modern-day demands regarding the peak-to-valley height of the surface, and consequently to avoid the absence of bits as indicated by missing magnetic material. The necessary peak-to-valley height cannot be achieved by polishing the magnetic layer, because subsequent processing is impossible. However, if magnetizable foils are used, they have to be available in the small thickness of approximately 0.05-0.8 .mu.m which is impossible in a practical application, for such thin foils are neither available nor producible.